A cracked rail outside Tenleytown means delays during the Friday evening rush hour for Metro's Red Line riders.

Crews are working to replace a 39-foot section of rail after it cracked in a tunnel outside the Tenleytown station. Trains single-tracked between the Friendship Heights and Van Ness stations starting around 3 p.m., causing 20-minute delays by 4:30 p.m.

Metro spokeswoman Caroline Lukas said she wasn't sure why the rail cracked and was still looking into it.

The cracks are an ongoing problem for the transit agency. Metro says usually occur during extreme cold or heat, when changes of temperature cause metal to expand or contract. Metro had 28 cracked rails in 2012 and 49 in 2011.